1. Field of the Invention
This invention relates to plastic articles, and more particularly relates to plastic containers of reduced gas permeation and method therefor.
2. Background
In the food, beverage and medical industries, thermoplastic polymers have come to the fore as preferred materials for manufacture of various articles and have largely replaced glass and metal. Polymer molecules are, however, large and the surfaces of shaped plastic articles are somewhat porous. As a result, plastic soft drink bottles require protection against loss of carbonation due to permeation of carbon dioxide through the container walls. On the other hand, food containers require protection against the ingress of air and water vapor and spoilage therefrom.
A recent approach to improving gas transmission properties for plastic containers is based on plasma deposition of a thin film of a silicon oxide (SiO.sub.x) material on the container surface. Representative disclosures of this technology may be found in U.S. Pat. No. 4,552,791 to Hahn, 5,047,131 to Wolfe et al. and EP published application 469,926 to Felts.
In the medical arts, plastic, in particular polyolefin, is used for the manufacture of a wide variety of articles, such as for example, syringes, flasks, vials and tubes. Many of these articles must be sterilized, after which they spend an indeterminate period of time on the shelf before use.
It is well known that conventional sterilization by high energy irradiation causes plastics, in particular polyolefins, to discolor and become embrittled upon exposure to high energy radiation at levels above 0.1 megarads. As a result, much effort has been expended toward additives which would stabilize polyolefins toward irradiation degradation. This technology is summmarized in U.S. Pat. No. 4,959,402 to Williams et al.
While the above disclosures have individually addressed the problems of reducing gas transmission and reducing polymer degradation, there is a definite need in the packaging and medical arts for radiation sterilized articles of still greater imperviousness to oxygen transmission. The present invention fulfills this need.